Zeeman's comparison theorem

As an illustration, we sketch the proof of Borel's theorem, which says the cohomology ring of a classifying space is a polynomial ring.[2]

First of all, with G as a Lie group and with ${\displaystyle \mathbb {Q} }$ as coefficient ring, we have the Serre spectral sequence ${\displaystyle E_{2}^{p,q}}$ for the fibration ${\displaystyle G\to EG\to BG}$. We have: ${\displaystyle E_{\infty }\simeq \mathbb {Q} }$ since EG is contractible. We also have a theorem of Hopf stating that ${\displaystyle H^{*}(G;\mathbb {Q} )\simeq \Lambda (u_{1},\dots ,u_{n})}$, an exterior algebra generated by finitely many homogeneous elements.

Next, we let ${\displaystyle E(i)}$ be the spectral sequence whose second page is ${\displaystyle E(i)_{2}=\Lambda (x_{i})\otimes \mathbb {Q} [y_{i}]}$ and whose nontrivial differentials on the r-th page are given by ${\displaystyle d(x_{i})=y_{i}}$ and the graded Leibniz rule. Let ${\displaystyle {}^{\prime }E_{r}=\otimes _{i}E_{r}(i)}$. Since the cohomology commutes with tensor products as we are working over a field, ${\displaystyle {}^{\prime }E_{r}}$ is again a spectral sequence such that ${\displaystyle {}^{\prime }E_{\infty }\simeq \mathbb {Q} \otimes \dots \otimes \mathbb {Q} \simeq \mathbb {Q} }$. Then we let

${\displaystyle f:{}^{\prime }E_{r}\to E_{r},\,x_{i}\mapsto u_{i}.}$

Note, by definition, f gives the isomorphism ${\displaystyle {}^{\prime }E_{r}^{0,q}\simeq E_{r}^{0,q}=H^{q}(G;\mathbb {Q} ).}$ A crucial point is that f is a "ring homomorphism"; this rests on the technical conditions that ${\displaystyle u_{i}}$ are "transgressive" (cf. Hatcher for detailed discussion on this matter.) After this technical point is taken care, we conclude: ${\displaystyle E_{2}^{p,0}\simeq {}^{\prime }E_{2}^{p,0}}$ as ring by the comparison theorem; that is, ${\displaystyle E_{2}^{p,0}=H^{p}(BG;\mathbb {Q} )\simeq \mathbb {Q} [y_{1},\dots ,y_{n}].}$

• McCleary, John (2001), A User's Guide to Spectral Sequences, Cambridge Studies in Advanced Mathematics, vol. 58 (2nd ed.), Cambridge University Press, ISBN 978-0-521-56759-6, MR 1793722
• Roitberg, Joseph; Hilton, Peter (1976), "On the Zeeman comparison theorem for the homology of quasi-nilpotent fibrations" (PDF), The Quarterly Journal of Mathematics, Second Series, 27 (108): 433–444, doi:10.1093/qmath/27.4.433, ISSN 0033-5606, MR 0431151
• Zeeman, Erik Christopher (1957), "A proof of the comparison theorem for spectral sequences", Proc. Cambridge Philos. Soc., 53: 57–62, doi:10.1017/S0305004100031984, MR 0084769